1. Field of the Invention
This invention relates to a device that multiplexes and demultiplexes light. In particular, it relates to a device that demultiplexes signal light of a particular wavelength from multi-wavelength light, or multiplexes light of a particular wavelength into multi-wavelength light.
2. Description of the Related Art
As the use of information processing technology spreads to many different fields, the amount of information transferred via networks is increasing. As this happens, many network transmission paths have come to be made of optical fiber to increase the distance, speed and amount of information transmitted.
In optical transmission systems, in recent years the wavelength division multiplexing transmission method has been attracting attention as a technology for transmitting large amounts of information. Wavelength division multiplexing transmission is a technology in which a plurality of signals to be transmitted are put onto carriers of different wavelengths and then the plurality of wavelengths are multiplexed for transmission through a single optical fiber. In a wavelength division multiplexing transmission system, in general, the plurality of wavelengths are respectively called channels.
In a wavelength division multiplexing transmission system, a device that extracts particular signals from the plurality of signals that are transmitted as wavelength multiplexed light, and a device that adds signals to be transmitted to wavelength multiplexed light, are necessary. In order to extract a particular signal from a plurality of multiplexed signals, a device that separates light of the wavelength that carries that particular signal from the wavelength multiplexed light is necessary. A device such as this is called a light demultiplexing device. Meanwhile, in order to add a signal that is to be transmitted to wavelength multiplexed light, a device that multiplexes light of the wavelength that carries the signal to be transmitted to the wavelength multiplexed light is necessary. Such a device is called a light multiplexing device.
FIG. 1 shows the basic configuration of a light multiplexing and demultiplexing device. This light multiplexing and demultiplexing device consists of the demultiplexing device 10 that demultiplexes light of wavelength .lambda.1 from wavelength multiplexed light consisting of the wavelengths .lambda.1, .lambda.2, . . . , .lambda.n; the multiplexing device 20 that multiplexes light of wavelength .lambda.1 into the wavelength multiplexed light; and the isolator 30 that is installed between the demultiplexing device 10 and the multiplexing device 20. The demultiplexing device 10 includes the circulator 11, and the fiber grating 12 that reflects light of wavelength .lambda.1; the multiplexing device 20 includes the circulator 21, and the fiber grating 22 that reflects light of wavelength .lambda.1.
The action of the demultiplexing device 10 is as follows. The circulator 11 guides wavelength multiplexed light that is input from the main transmission path 41 to the fiber grating 12. Among the plurality of wavelength components included in the wavelength multiplexed light, the fiber grating 12 reflects light of wavelength .lambda.1 to the circulator 11, and outputs light of wavelengths other than .lambda.1 to the isolator 30. The circulator 11 guides light of wavelength .lambda.1 reflected by the fiber grating 12 to the branch transmission path 43. In this way, the demultiplexing device 10 extracts light of wavelength .lambda.1 from the multi-wavelength light and outputs it to the branch transmission path 43.
The action of the multiplexing device 20 is as follows. When wavelength multiplexed light is received from the isolator 30, the fiber grating 22 guides that wavelength multiplexed light to the circulator 21. However, light of wavelength .lambda.1 does not pass through the fiber grating 22. The circulator 21 guides the wavelength multiplexed light received from the fiber grating 22 to the main transmission path 42. In addition, when light of wavelength .lambda.1 is received from the branch transmission path 44, the circulator 21 guides that light to the fiber grating 22. When light of wavelength .lambda.1 is received from the circulator 21, the fiber grating 22 reflects that light to the circulator 21. Then the circulator 21 guides the light of wavelength .lambda.1 that was reflected by the fiber grating 22 to the main transmission path 42. In this way, the multiplexing device 20 multiplexes the light of wavelength .lambda.1 received from the branch transmission path 44 into the wavelength multiplexed light received from the isolator 30, and outputs the resulting wavelength multiplexed light to the main transmission path 42.
FIG. 2A shows the configuration of a light demultiplexing device that extracts light of a plurality of wavelengths from wavelength multiplexed light. The configuration shown here extracts light of wavelengths .lambda.1 and .lambda.n from wavelength multiplexed light including the wavelengths .lambda.1, .lambda.2, . . . , .lambda.n. In FIG. 2A, the circulator 11 and the fiber grating 12 are the same as the corresponding items shown in FIG. 1. The fiber grating 13 is a reflecting element that reflects light of wavelength .lambda.n.
The action of the light demultiplexing device shown in FIG. 2A is as follows. The circulator 11 guides wavelength multiplexed light input from the main transmission path 41 to the fiber grating 12. Then, as was explained with reference to FIG. 1, the fiber grating 12 reflects light of wavelength .lambda.1, from among the plurality of wavelength components included in the wavelength multiplexed light, to the circulator 11. Then the circulator 11 guides the light of wavelength .lambda.1 reflected by the fiber grating 12 to the branch transmission path 43.
Meanwhile, the wavelength multiplexed light that has passed through the fiber grating 12 is guided to the fiber grating 13. When this wavelength multiplexed light is received, the fiber grating 13 reflects the light of wavelength .lambda.n to the fiber grating 12, while light of wavelengths other than .lambda.n is output to the transmission path 45. The light of wavelength .lambda.n that was reflected by the fiber grating 13 passes through the fiber grating 12 and is guided to the circulator 11, and then is guided by the circulator 11 to the branch transmission path 43.
In this way, the light demultiplexing device shown in FIG. 2A demultiplexes light of wavelength .lambda.1 and light of wavelength .lambda.n from the wavelength multiplexed light, and outputs them to the branch transmission path 43.
In order to multiplex light of 2 or more different wavelengths into wavelength multiplexed light, it is sufficient to add a plurality of fiber gratings that reflect the respective plurality of wavelengths to the multiplexing device 20 shown in FIG. 1; the configuration is basically the same as that of the light demultiplexing device shown in FIG. 2A.
A fiber grating reflects light in a particular wavelength band. However, in general, the reflectivity is less than 100%, even within the reflected wavelength band. The reflection characteristics of a fiber grating are shown in FIG. 3A. The transmission characteristics of the same fiber grating are shown in FIG. 3B.
A fiber grating has, for example, a reflection wavelength band on the order of 1 to 2 nm. When the fiber grating receives light of a wavelength within this reflection wavelength band, it reflects that light. The loss in this reflection should ideally be 0, but in fact a loss of x (dB) occurs as shown in FIG. 3A. If the fiber grating receives light of a wavelength outside of the reflection wavelength band, ideally that light should not be reflected at all. However, in practice even for light of wavelengths outside of the reflection wavelength band there is reflection accompanied by a loss of x+y (dB).
Similarly, if a fiber grating receives light of a wavelength within the reflection wavelength band, ideally none of that light should be transmitted. However, in practice that light is transmitted with a loss of a+b (dB) as shown in FIG. 3B. When the fiber grating receives light of a wavelength that is outside of the reflection wavelength band, that light is transmitted. The loss in this transmission should ideally be 0, but in fact a loss of a (dB) occurs.
Thus, even when light is within the reflection wavelength band the fiber grating transmits part of it, and even if light is outside of the reflection wavelength band the fiber grating reflects part of it. This in turn sometimes causes coherent cross-talk in a light multiplexing and demultiplexing device. For example, in the light demultiplexing device shown in FIG. 2A, light of wavelength .lambda.n can be attenuated by coherent cross-talk. Referring to FIG. 2B, let us explain the coherent cross-talk in the light demultiplexing device shown in FIG. 2A.
As shown by path (1), light of wavelength .lambda.1 is basically reflected by the fiber grating 12, which is the reflective element used for .lambda.1, and guided to the branch transmission path 43. This reflected light is the signal light. Hereafter, this light will be referred to as the "signal light A1". The loss that occurs in this path is only the reflection loss (x) in the fiber grating 12.
However, as explained above, the fiber grating 12 transmits part of the light of wavelength .lambda.1. As shown by path (2), part of the light that has passed through the fiber grating 12 is reflected by the fiber grating 13, which is the reflective element for .lambda.n, and guided to the branch transmission path 43. This reflected light is the interference light with respect to the signal light A1. Hereafter, this reflected light will be called the "interference light B1". In this path (2), reflection loss (x+y) occurs in the fiber grating 13 and transmission loss (2a+2b) occurs in the fiber grating 12.
As shown by path (3), part of the light of wavelength .lambda.1 that is reflected by the fiber grating 13 is further reflected by the fiber gratings 12 and 13, then is transmitted through the fiber grating 12 and output to the branch transmission path 43. This light is also interference light with respect to the signal light A1. Hereafter, this reflected light will be called "interference light C1". In path (3), in addition to the loss in path (2), there is an additional reflection loss (x) in the fiber grating 12 and reflection loss (x+y) in the fiber grating 13.
There is also some light which, after further passing repeatedly between the fiber gratings 12 and 13 is transmitted through the fiber grating 12 and guided to the branch transmission path 43, but the power of this light is very small, so it is neglected.
The respective powers of the signal light A1, the interference light B1 and the interference light C1 that are output to the branch transmission path 43 are as follows, where P is the power of the light of wavelength .lambda.1 input to this light demultiplexing device, and the loss in the circulator 11 is neglected:
signal light A1: P-x PA0 interference light B1: P-2(a+b)-(x+y) PA0 interference light C1: P-2(a+b)-2(x+y)-x PA0 signal light An: P-2a-x PA0 interference light Bn: P-(x+y) PA0 interference light Cn: P-2(a+x)-(x+y)
The magnitude of the effect of the interference light with respect to the signal light depends on the difference between the power of the signal light and the power of the interference light; the smaller that power difference the greater the attenuation of the signal light by the interference light. The power difference between the signal light A1 and the interference light B1 is 2(a+b)+y; the power difference between the signal light A1 and the interference light C1 is 2(a+b)+2(x+y). Here, as shown in FIGS. 3A and 3B,the reflection loss y and the transmission loss b are large (in a typical fiber grating, y and b are both on the order of 20 dB), so the signal light B1 and C1 are sufficiently small relative to the signal light A1. Consequently, the attenuation of the signal light .lambda.1 is slight. That is to say, attenuation of a signal transmitted using light of wavelength .lambda.1 is small.
Meanwhile, as shown by path (2), light of wavelength .lambda.n is basically reflected by the fiber grating 13 that is the reflective element used for .lambda.n and output to the branch transmission path 43. This reflected light is the signal light. Hereafter, this reflected light will be called "signal light An". In this path, reflection loss (x) in the fiber grating 13 and transmission loss (2a) in the fiber grating 12 occur.
However, as discussed above, the fiber grating 12 reflects part of the light other than light of wavelength .lambda.1. That is to say, as shown by path (1), part of the light of wavelength .lambda.n is reflected by the fiber grating 12, which is the reflective element used for .lambda.1, and guided to the branch transmission path 43. This reflected light becomes the interference light with respect to the signal light An. Hereafter, this reflected light will be called the "interference light Bn". In this path, reflection loss (x+y) occurs in the fiber grating 12.
Further, as is shown by path (3), part of the light of wavelength .lambda.n that is reflected by the fiber grating 13 is further reflected by the fiber gratings 12 and 13, then passes through the fiber grating 12 and is output to the branch transmission path 43. This reflected light also becomes interference light with respect to the signal light An. This reflected light will be called the "interference light Cn". In path (3), in addition to the loss in path (2), there is an additional reflection loss (x+y) in the fiber grating 12, and a reflection loss (x) in the fiber grating 13.
Consequently, the respective light powers of the signal light An, the interference light Bn and the interference light Cn that are output to the branch transmission path 43 are given as follows, where P is the power of the light of wavelength .lambda.n that is input to this light demultiplexing device and the loss in the circulator 11 is neglected.
Here, the reflection loss x and the transmission loss a (these are called dead losses, and are on the order of 0.2 dB in a typical fiber grating) are sufficiently small with respect to the reflection loss y and the transmission loss b, and so can be neglected, in which case we see that the power difference between the signal light An and the interference light Bn, and the power difference between the signal light An and the interference light Cn, are both on the order of y.
Thus, the power difference between the signal light and the interference light in the light of wavelength .lambda.n is small compared to the power difference between the signal light and the interference light in the light of wavelength .lambda.1, which makes it that much easier for the signal light to be attenuated.
As in the case in which light of wavelength .lambda.n is demultiplexed, if we assume that the power differences between each interference light (the interference light Bn and Cn) and the signal light (the signal light An) are respectively y dB, then the power difference between the signal light An and the interference light Bn and Cn becomes y-3 dB. In addition, in the case in which the state of polarization of the signal light and the states of polarization of each interference light are in agreement, and in the case in which the electric field of the signal light agrees with the electric fields of each interference light, it becomes easier for the signal light to be attenuated by the interference light. The deterioration in this case is on the order of a maximum of about 3 dB in each respective case, when converted into a power difference between the signal light and the interference light. Consequently, in the case in which light of wavelength .lambda.n is demultiplexed using the light demultiplexing device shown in FIG. 2A, in the worst case the power difference between the signal light and the interference light becomes on the order of y-9 dB.
The problems discussed above do not occur only when a plurality of wavelength components are demultiplexed from the wavelength multiplexed light; they can also occur when a plurality of wavelength components are multiplexed into wavelength multiplexed light.
Thus, in an existing type of light multiplexing and demultiplexing device, in a case in which a plurality of signal light beams are multiplexed or demultiplexed using a plurality of reflecting elements, it is possible that the signal light will be attenuated by the reflection characteristics of the reflecting elements and by multiple reflections among the reflecting elements.